Child Support Letter for 13 year old

I live in Mass received a letter from DOR stating that an ex girlfriend has filed for child support. We are going to have to take a paternity test to see if the child is mine but I am wondering how child support is going to be figured if I am indeed the father. I have custody of my two older children from a previous relationship and receive about $125 a week in support for them. I am also married and my wife and I have two more children together so we have 4 children in our home that I am supporting. Will DOR take that into consideration especially since I had know idea about this other child. I will also have to supple insurance for this new child which I do get through my work but will that be considered as well? I am planning on consulting and attorney as soon as we find out the test results.